1. Field of the Invention
The present invention relates to electron guns for use in cathode ray tubes and, more particularly, to an improved electron gun configuration enabling enhanced image resolution on the screen of a cathode ray tube.
2. Description of the Related Art
Cathode ray tubes (CRTs) have been used since the early 1900's to project images onto screens and are commonly used today in computer terminals and television sets. A CRT utilizes a vacuum tube in which cathode rays, usually in the form of narrow beams, are projected onto a fluorescent screen to produce a luminous spot on the screen. Cathode rays are produced by an electron gun which propels a stream of electrons toward the screen via a strong electric field created inside the vacuum tube.
The typical electron gun includes a cathode, a control grid and an anode. A thermionic heater raises the temperature of the emitting surface of the cathode to a point at which thermionic electron emission can occur. By applying a large voltage differential between the cathode and the anode, the electrons can be drawn from the cathode emitting surface into the projected beam. Application of a negative voltage to the control grid between the cathode and the anode causes the beam to shut off. By modulating the voltage on the control grid, the beam intensity can be varied.
It has long been desired within the art to improve the resolution of an image visible on the CRT screen. The image is formed by numerous spots projected onto the screen. If each individual spot size could be decreased, a greater number of spots per unit area of screen can make up a particular image, thus increasing its sharpness or resolution. Accordingly, most efforts to improve CRT screen resolution center on reducing the spot size.
A problem which consistently hinders spot size reduction is beam spreading. When the electron gun emits the beam, the path of the electrons tends to diverge. This divergence, or beam spreading, causes a wider spot than desired to be projected onto the screen.
Various techniques are employed to reduce the beam spreading phenomenon. One such technique involves the use of apertures to filter or clip the outer diameter of the beam. The aperture would comprise a small hole placed in a charged plate positioned between the electron gun and the screen. As the beam passes through the aperture, the diverging electrons strike the edges of the aperture, and are shunted off. Apertures are effective in reducing the diameter of the beam, resulting in a smaller spot size being projected onto the screen.
Although aperture usage has a beneficial effect on resolution, it has an associated drawback with regard to brightness. By shunting off electrons from the beam, the overall intensity of the beam is reduced since fewer electrons remain in the beam which ultimately reaches the screen to form the spot. As a result, the brightness of the spot is reduced. To remedy the degraded brightness, the voltage on the control grid can be increased, or driven more positive relative to the cathode. However, the conventional wisdom in the art of electron guns is that an increase in the drive voltage would further degrade the resolution on the screen, since greater beam spreading would result. Thus, CRT electron gun designers were required to strike a balance between resolution and brightness, in that both parameters could not be maximized simultaneously.